Their hope was that the neutrons shooting into the acetone would collide with the carbon and hydrogen nuclei, and this would create disturbances that would "seed" the bubbles produced by the sound waves. Many more bubbles than normal would be formed at once, and on average the bubbles would grow much larger than usual before they collapsed. Perhaps, the scientists thought, the bubbles would get so big that their collapse would produce temperatures near 10 million degrees¿hot enough to cause a few deuterium atoms in the acetone to fuse into helium or tritium (hydrogen with two extra neutrons).
Image: courtesy of RUSI TALEYARKHAN
Creating even small numbers of fusing atoms would be a big deal. Fusion reactions release lots of energy, hence their usefulness for lighting stars and making mushroom clouds. The energy comes out in the form of neutrons humming along at 2.5 million electron volts (MeV), fast-moving protons and hot tritium and helium atoms. When the Taleyarkhan group checked the samples for tritium, the researchers found that it had indeed increased¿but only in the deuterium-laced acetone that had been zapped with both sound and neutrons. Tritium levels didn¿t change significantly in normal acetone put through the process, nor in deuterated acetone shot just with neutrons or subjected only to a good ringing.
They also looked for neutrons emerging from the flask after the neutron gunshot had dissipated and the bubbles had burst. Sure enough, their scintillation detector started scintillating about twice as fast within a few microseconds of the strongest sonoluminescent flashes. Working through a complicated set of calculations, the researchers reckoned that they observed a four-percent increase in 2.5 MeV neutrons just after the onset of bubble formation. That is certainly not enough to start a chain reaction (thank goodness), or even enough to produce as much energy as the apparatus consumes. But if it were confirmed, it would be an entirely new approach to generating fusion energy.
The Race to Test the Results
Unsurprisingly, many research groups around the world are scrambling to try this out for themselves. But the only one to make a report so far has disputed on several technical grounds the evidence that any atoms were fusing, though the group did allow that something strange was going on. Dan Shapira and Mike Saltmarsh, the group's leaders, had been asked last May by science managers at ORNL to check the Taleyarkhan group¿s findings.
Shapira and Saltmarsh brought in a different kind of neutron detector that is 30 times the size of the scintillator that the first team used. (A bad idea, Taleyarkhan complained in a rebuttal, because it is more likely to pick up background radiation and to overload the electronics.) The new detector system was triggered by a neutron or gamma-ray strike, and then matched that to any sonoluminescent flash that happened within 10 microseconds before or after the strike. (But that dilutes the signal, because neutron/gamma hits are much more common than flashes, complains the Taleyarkhan group, whose detectors worked the other way around.)